1. Field of the Invention
The present invention relates to sheet-like polyethylene terephthalate materials having slight surface roughness, a process for their preparation by exposure of the surfaces to UV radiation produced by the decomposition of excimers and the use of polyethylene terephthalate films treated in this manner as substrate material for ferromagnetic thin metal layers.
The embodiment of sheet-like polyethylene terephthalate material having a defined surface roughness is required for many uses of this material, The roughness generally results in an improvement in the adhesion, for example in adhesive bonds, and in the printability or the wettability. A toughened surface is also advantageous in the stacking of this material or in the winding of corresponding film webs. Polyethylene terephthalate (PET) films having a defined roughness are of considerable importance in particular for the production of magnetic recording media which consist of a polymeric substrate, a coherent, ferromagnetic thin metal layer applied to the surface of the substrate by a PVD (physical vapor deposition) method and, if required, a protective layer formed on the metal layer. The surface roughness of the films ensures little wear of the magnetic layer in the case of tribological stress due to the head. The roughening of the polymer surface also results in an improvement in the adhesion between the polymer and the magnetic layer. In addition, the mechanical and chemical stability of the metal layer set particular requirements for the procedure. However, these problem areas cannot be viewed in isolation, since any optimization of the mechanical properties of the magnetic recording media under discussion must never be achieved at the expense of the magnetic properties, i.e. of the properties relating to information storage.
A reduction in the wear of the magnetic layer due to the tribological stress of the head is achieved, inter alia, by virtue of the fact that direct contact between the head and the metal layer is avoided over its entire, macroscopic contact surface by means of the surface roughness. In order for this effect to be achieved, it is important for the average distance between the protuberances on the surface to be small compared with the dimensions of the macroscopic contact surface of the head. The height of the surface roughness is limited by the fact that, when the roughness has an excessively great height, the magnetic recording and playback properties of the applied layers are poor owing to the attenuation due to distance.
2. Description of the Related Art
Films in which a defined surface roughness is produced by incorporation or application of very small particles of an inert material are being used to date in order to obtain the desired properties of the polymer films for use as substrate material for magnetic thin-layer media (e.g. European Patent 0,153,853). Although these films are suitable, the production of such films requires particular and expensive production techniques.
The other conventional processes for the surface treatment of polymer surfaces in plasma, or by glow discharge, corona discharge, flaming, chemical etching or ion irradiation prior to metallization are never completely satisfactory. Essential aspects here were in particular the inadequate controllability of the energy effect and/or residual gas control and the resulting contamination by decomposition products.
The processes described to date and involving UV irradiation of polymer surfaces with continuous UV lamps, for example mercury vapor lamps, for improving the coatability, for improving the printability of polyolefins (U.S. Pat. No. 4,933,123) and for increasing the adhesive strength of adhesives on polyethylene terephthalate films (JP-A 313 850/1989), on the other hand, also results in only an insufficient increase in adhesion in the case of magnetic recording media having coherent metal layers. Owing to the long exposure time to continuous UV lamps, of the order of a few minutes, the process is very time-consuming and does not permit high processing speeds.
Furthermore, it is known that a periodic nub-like or cylindrical structure can be produced on the surface of oriented PET films by exposure to a UV excimer laser (E. Ahrenholz et al., Appl. Phys. A 53 (1991), 330). These structures occur in an energy range in which the laser radiation leads to removal of material (laser ablation). The structures have a typical distance of a few .mu.m and are completely formed only after irradiation of the films with their plurality of pulses (at least 3 or 4). The spacing of the structure as well as the height of the structures increase with increasing energy and number of pulses. The height of these structures may be several .mu.m. Both the height of these structures and the spacing of the structure are generally too large to permit the films treated in this manner to be used as substrates for magnetic thin-layer media.